Control circuit for an ignitron utilizing a silicon controlled rectifier and a zener diode in series



Dec. 24, 1968 H J G CONTROL CIRCUIT FOR AN IGNITRON UTILIZING A SILICON v CONTROLLED RECTIFIER AND A ZENER DIODE IN SERIES Filed Sept. 10, 1964 INVENTOR HENRI J. G.M. VAN DAELEN AGENT .M.. VAN DAELEN r 3,418,525

United States Patent 3,418,525 CONTROL CIRCUIT FOR AN IGNITRON UTILIZ- ING A SILICON CONTROLLED RECTIFIER AND A ZENER DIODE IN SERIES Henri Jean Gerard Marie van Daelen, Emmasingel, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Sept. 10, 1964, Ser. No. 395,384 Claims priority, application Netherlands, Sept. 12, 1965, 297,856 5 Claims. (Cl. 315168) ABSTRACT OF THE DISCLOSURE An ignitron ignition circuit includes a Zener diode and silicon controlled rectifier connected in series across the anode and ignitor electrodes of the ignitron. The Zener diode is arranged to automatically extinguish the ignitron priming are between ignitor and cathode once the main discharge are between the anode and cathode occurs.

The invention relates to a circuit arrangement for igniting an ignitron. One object of the invention is to replace the conventional thyratrons or other controllable gas or vapour-filled tubes and/or other circuit elements by a controllable semiconductor rectifier or other rectifying semiconductor switch.

The ignition of an ignitron requires the supply of a fairly large forward current pulse to the ignition electrode during the time that its anode is polarized positively. An arc is then first struck between the ignition electrode and the cathode, which are ionizes the gas or the vapour above the cathode and thus ignites the main discharge between the anode and the cathode of the ignitron. The arc between the ignition electrode and the cathode must extinguish a very short time thereafter. In the conventional arrangements this is brought about without difficulties with the aid of a thyratron. A positive voltage, usually the same as applied to the anode of the ignitron, or a voltage derived therefrom, is applied to the ignition electrode of the ignitron through the anode-cathode-electrode path of the thyratron at an instant when a positive control pulse is applied to the control electrode of the thyratron. After the main discharge between anode and cathode of the ignitron has been ignited, the discharge are between the ignition electrode and the cathode automatically extinguishes because the arc voltage between anode and cathode of the ignitron is much lower than the sum of its arc voltage between ignition electrode and cathode and of the arc voltage between anode and cathode of the thyratron.

In an ignitron arrangement, it seems attractive to replace the thyratrons and/ or other circuit elements hitherto employed by controllable semiconductor rectifiers or other rectifying semiconductor switches. The latter ar considerably more rugged and occupy considerably less space than thyratrons. Furthermore the additional transformer for heating the cathode of the thyratron can be eliminated resulting in greater circuit economy. When simply replacing the ignition thyratron of an ignitron by a correspondingly connected, controllable semiconductor rectifier, the difficulty has arisen that the are between the ignition electrode and the cathode of the ignitron is not always automatically extinguished after the ignition of the main discharge are between anode and cathode. This is due to the fact that the voltage drop across the collectoremitter-electrode path of the controllable semiconductor rectifier does not sufliciently augment the arc voltage between ignition electrode and cathode of the ignitron to ensure the extinction of this arc at the desired instant.

The present invention provides a circuit arrangement by means of which said ditficulty can be overcome. The arrangement according to the invention features an element having a strongly voltage-dependent resistance, and which only becomes highly conducting when the Voltage across it exceeds the anode-cathode arc voltage of the ignitron, connected in series with the main current electrode path of the rectifying semiconductor switch.

Said element may be a voltage-dependent resistor (VDR). It is preferably a semiconductor diode connected in the reverse direction and having a Zener voltage exceeding the anode-cathode arc voltage of the ignitron reverse direction.

The invention will be described 'more fully with reference to an embodiment.

The very simple embodiment comprises a supply transformerhaving a primary winding connected to the AC lines. The secondary winding is connected to the series combination of the anode-cathode electrode path of an ignitron 3 and of a load 4. The anode of the ignitron is directly connected to one end of the secondary winding of said transformer. In order to ignite the ignitron, a positive voltage is applied to its ignition electrode through the collector-emitter electrode path of a controllable semiconductor rectifier 5 of the n-p-n-p-type. This voltage should at the most be equal to that which is simultaneously applied to the anode of the ignitron (for example 220 v.). It is derived from the same end of the secondary winding of the supply transformer to which the anode of the ignitron is connected. Alternatively, it may be derived from a tapping of said secondary winding.

In accordance with the invention, a semiconductor diode having a Zener voltage (of for example 25 v.) exceeding the anode-cathode arc voltage (for example 20 v.) of the ignitron is connected in the reverse direction, in series with the collector-emitter electrode path of the controllable semiconductor rectifier. The emitter of the conductivity-type of the controllable rectifier 5 is directly connected to the ignition electrode of the ignitron. Its collector is connected to the anode of the diode 6 and the cathode of the diode is connected to the anode of the ignitron.

During each positive half period of the voltage applied to the ignitron anode, the ignitron ignites at an instant determined by a positive pulse applied shortly before between the control-electrode and the emitter of the controllable semiconductor rectifier. This pulse renders the recitfier conducting, which rectifier was blocked up to that instant by its own threshold voltage between emitter and control-electrode or by a reverse polarising voltage. If the voltage operative at this instant between anode and cathode of the ignitron has an adequate value and if it exceedes, in particular, the Zener voltage of the semiconductor diode, the latter breaks down and the full anode voltage reduced by the said Zener voltage and the voltage drop of a few tens of volts across the controllable semiconductor rectifier is applied to the ignition electrode of the ignitron. An arc is thus struck between ignition electrode and cathode of the ignitron. At the surface of the mercury cathode, this arc soon produces a hot cathode spot, so that the mercury vapour in the ignitron is ionised. A very short time after the application of the aforesaid positive control-pulse, an are having an arc voltage of 20 v. is struck between anode and cathode of the ignitron owing to said ionisation. Said arc shortcircuits so to say the discharge path via the semiconductor diode, the controllable semiconductor rectifier and the ignition electrode-cathode path of the ignitron, so that the discharge through said path is immediately extinguished. The discharge in a reverse direction through the semi-conductor diode is first interrupted, and brings about the extinction of the are between ignition electrode and cathode of the ignitron and the return of the controllable semiconductor rectifier into the cut-off state.

What is claimed is:

1. An ignitron ignition circuit comprising, a source of AC voltage, a load, means connecting the anode-cathode path of said ignitron in series with said load across said voltage source, a semiconductor controlled rectifier having first and second electrodes defining a main current path therein and a control electrode for controlling the current fiow in said main current path, a voltage sensitive resistance element which exhibits a given threshold voltage at which it becomes highly conductive, said threshold voltage being greater than the anode-cathode arc dicharge voltage of the ignitron, means connecting said voltage sensitive element, said first and second electrodes of the controlled rectifier, and the ignitor electrode and cathode of the ignitron in a series circuit, means for applying to said series circuit a positive voltage of a magnitude at the most equal to the magnitude of the AC voltage applied to the ignitron anode, and means for applying a control voltage to the control electrode of said controlled rectifier.

2. A circuit as defined in claim 1 wherein said voltage applying means includes an AC voltage supply of the same frequency and in phase with said source of AC voltage.

3. A circuit as defined in claim 1 wherein said voltage sensitive element comprises a Zener diode having a Zener breakdown voltage that exceeds the anode-cathode arc discharge voltage of the ignitron.

4. An ignitron ignition circuit comprising, a source of AC voltage, a load, means connecting the anode-cathode path of said ignitron in series with said load across said voltage source, a semiconductor controlled rectifier having first and second current path defining electrodes and a control electrode, a Zener diode having a threshold voltage that is greater than the anode-cathode arc discharge voltage of the ignitron, means connecting said Zener diode, said first and second electrodes of the controlled rectifier, and the ignitor electrode and cathode of the ignitron in a series circuit, means for applying a portion of said AC voltage across said series circuit to strike an are between the ignitor electrode and cathode of said ignitron, said Zener diode being controlled by the anodecathode voltage of the ignitron so as to block the flow of current in said series circuit as a function of said voltage, and means for applying a control voltage to the control electrode of said controlled rectifier that determines the ignition point of the arc discharge in the AC cycle.

5. An ignitron ignition circuit comprising, a source of AC voltage, a load, means connecting the anode-cathode path of said ignitron in series with said load across said voltage source, a semiconductor controlled rectifier having first and second current path defining electrodes and a control electrode, a Zener diode having a threshold voltage that is greater than the anode-cathode arc discharge voltage of the ignitron, means connecting said Zener diode and said first and second controlled rectifier electrodes in series circuit between the anode and ignitor electrodes of said ignitron so that said Zener diode determines the condition for blocking current flow in said series circuit as a function of the ignitron anode voltage, and means for applying a control voltage to the control electrode of said controlled rectifier.

References Cited UNITED STATES PATENTS 2,909,705 10/1959 Husson 315203 X 3,030,550 4/1962 Smeltzer 315 273 X 3,176,159 3/1965 Laishley 307-88.5 3,242,416 3/1966 White 30788.5 3,089,998 5/1963 Reuther 32322 3,194,987 7/1965 Mandel 30788.5 3,254,236 5/1966 Mang 30788.5

JAMES w. LAWRENCE, Primary Examiner.

C. CAMPBELL, 111., Assistant Examiner.

U.S. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,418,525 December 24, 1968 Henri Jean Gerard Marie van Daelen It is certified that error appears in the above identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading to the printed specification, line 10, "Sept. 12, 1965" should read Sept. 12, 1963 Signed and sealed this 24th day of March 1970.

(SEAL) Attest:

Edward M. Fletcher, Jr.

Attesting Officer Commissioner of Patents WILLIAM E. SCHUYLER, JR. 

